1. Field of the Invention
This invention relates generally to frequency agile resonant components, such as filters, resonators and antennas, and more particularly to a system for tuning such components to a target frequency.
2. Background of the Invention
The bandwidth of resonant components, such as antennas and certain types of filters, is critically dependent on size. A narrowband antenna can be made much smaller than an antenna of wider bandwidth. Since satellite communication systems operate at different transmit and receive frequencies, for example 1650 MHz transmit, and 1550 MHz receive, antennas must have sufficient bandwidth to cover both transmit and receive frequencies. As a result, a typical patch antenna covering both frequency bands, for example, needs to be over 2 inches in diameter, whereas a similar antenna covering only one of the frequencies of interest (i.e. part of one band) can be made under one inch in diameter.
There are four basic types of tuning for frequency agile components: mechanical, electronic, magnetic and electric. Mechanically tuned components typically extend or contract one or more of their physical resonant dimensions to vary the resonant frequency. Electronically tuned components typically use electronic devices connected directly to the component to modify the resonant frequency. Magnetically tuned components typically use magnetic fields to vary the permeability of the component, which is typically made of a ferrite material. The change in permeability changes the effective electrical dimension, or value, of the component, thereby varying the resonant frequency. Electrically tuned components typically use electric fields to vary the permittivity of the component, which is typically made of a ferroelectric material. The change in permittivity changes the effective electrical dimension of the component, or value, thereby varying the resonant frequency.
Common examples of frequency agile components include filters, resonators and antennas. In the prior art, the frequency agile component was considered to be a system on its own. This lead to carefully calibrated open loop systems. The effect of the control mechanism on resonant frequency had to be well known, as well as the effect of temperature, and the presence of objects in the reactive nearfield, aging, etc., which could not always be predicted, for example, a hand near the antenna. The communications device would simply adjust the control signal to the value from a look-up table (or equivalent) that corresponded to that frequency. The quality of the input match would be unknown, thereby providing no guarantee that the component was properly tuned.
For mobile communications equipment, tuning error can result in permanent loss of contact. The more narrowband the component is, the more critical the tuning precision, and consequently such systems are unsuitable for using in communications systems with different transmit and receive frequencies.
A closed loop method for component tuning is known that involves the use of a received signal strength indicator (RSSI). The system tunes the component to maximize the RSSI value. For systems where the transmit frequency is not the same as the receive frequency, this technique is not available, as the component can not be tuned for transmitting. Even in a receive-only, or shared frequency system, if the communications device is out of coverage or blocked, the component would not be tuned. With the component detuned, the communications device might never lock on to the receive signal again, or take an excessively long time to do so. Furthermore, as with other methods, the quality of the input match would be unknown.
U.S. Pat. No. 6,097,263 describes a closed loop tuning system for resonant cavities wherein the resonant frequency of the cavity is sensed and an electric device in the cavity is altered until the desired resonant frequency is attained. Such a device is not suitable for antennas since they are radiating into free space. Furthermore, a system as described in U.S. Pat. No. 6097263 would not be suitable for integration within a wireless transceiver. Finally, emissions specifications are not addressed in the invention disclosed by U.S. Pat. No. 6097263.
According to the present invention there is provided a tunable resonant system, comprising an electric element; a core having a controllable parameter that determines the resonant frequency of the system; a frequency generator for supplying a low power, narrowband signal at a selectable frequency to said electric element; an arrangement for measuring the reflected or transmitted power of said applied narrowband signal; and a controller for adjusting the value of said controllable parameter to vary the resonant frequency of the system in a closed loop until the reflected power is at a minimum.
Typically, the resonant system is an antenna, such as a patch antenna suitable for satellite communications, but the invention is also applicable to other resonant systems, such as filters and resonators. While it is possible to measure the transmitted power, measurement of the reflected power is preferred.
In systems that have different transmit and receive frequencies, the invention permits the use of an antenna of bandwidth that merely needs to be sufficient to accommodate one of the transmit and receive frequencies at a time. This permits a significant reduction in the physical size of the antenna. An antenna having a diameter in the order of one inch is suitable to accommodate transmit and receive frequencies at 1550 MHz and 1650 MHz.
An additional advantage of the invention is that the narrowband antenna can in itself act as a filter tuned to the carried frequency of the transmit or receive signal and thereby simplify the front-end RF electronics of the transmitter and receiver.
This invention eliminates the division between the frequency agile component and the communications device. The electronics used in the communications device are reused to form a closed loop frequency tuning system for the component.
The component is tuned to the required frequency in a guard time immediately prior to a transmission or reception.
The invention has the advantage that the need for highly accurate and detailed calibration is eliminated because of the error tolerant nature of the closed loop tuning scheme. The hardware required to tune the component reuses existing electronics in the communications device. An open loop system based on a simple calibration is used to accelerate the tuning process. Furthermore, the quality of the input match is known. Additionally, the method is not dependant on being within network coverage since the signal used to tune the antenna is generated locally.
The invention automatically accounts for temperature variation since the resonant frequency is found for any particular set of conditions. In the prior art, heaters were used eliminate temperature variation, and such heaters are not required with the present invention.
The invention also provides a method of tuning a resonant system including an electric element and a core having a controllable parameter that determines the resonant frequency of the system, comprising supplying a low power, narrowband signal at a selectable frequency to said electric element; measuring the power of said applied narrowband signal that is reflected or transmitted from said electric element; and adjusting the value of said controllable parameter to vary the resonant frequency of the system in a closed loop until the reflected power is at a minimum.